Glasses based on the elements sulfur, selenium and tellurium have been formulated and produced as infrared optical materials for many years. The emphasis has usually been to identify a composition with good optical properties and with favorable physical properties as well. Desirable features of such glasses include strength, hardness, high softening points, low thermal expansion and resistance to thermal shock. The desire is to have sturdy optics for application in systems to be used in the field for example by the military.
However, different applications require glasses with different properties. For example, glasses to be drawn into fibers should not be brittle. Experience has shown that the strongest high softening glasses do not make the best fiber, with fused pure silica being an exception. It is desirable to have a lower softening glass because the drawing equipment is easier to fabricate and has a longer life. The same may be said about molding or extruding glass. The processes carried out at lower temperatures are easier and less expensive. The molds or extruders have a longer life operating at lower temperature. Of course the glass must have a softening point that meets the requirement of the application.
The success of the pure fused silica fiber is well known. The success for infrared transmitting chalcogenide glass fibers is not nearly as good. Chalcogenide glasses are much weaker. Drawn fibers are only a fraction as strong, about one seventh, in comparison to silica fibers. Infrared optical absorption is orders of magnitude greater in comparison to silicates. There are a number of good infrared lasers that emit watts of energy in the infrared. It would be desirable to have a flexible infrared transmitting glass fiber capable of transmitting that energy from the laser to an inaccessible location such as in the case of use for surgery. The carbon dioxide laser which emits intense infrared radiation continuously at 10.6 micrometers, has been approved for surgical use by the FDA. However, suitable flexible glass fiber suitable for this purpose has not been found.
Because the thermal change in refractive index for the glass used to make the fiber has an appreciable positive magnitude, as the energy flows through the fiber, the phenomena “thermal lensing” occurs. Thermal lensing may be best described as self focusing within a solid brought about by a radial change in refractive index about the center ray of a transmitted beam due to absorption, generated heat and the thermal change in refractive index for the material. Glasses are particularly susceptible because their disordered structure leads to low thermal conductivity. The absorbed radiation heats up the fiber in a non uniform manner producing a lensing action in the fiber which focuses the laser energy burning the fiber into. The only exception to this occurrence was fiber made from arsenic trisulfide glass. It has been reported that over 70 watts of laser power from a carbon monoxide laser emitting continuously at 5.4 Micrometers was transmitted through an arsenic trisulfide fiber 400 micrometers in diameter without failure. The thermal change in refractive index for the glass at that wavelength was zero, so no thermal lensing occurred.
A need has arisen for a composition that has a zero thermal change in refractive index resulting in a glass suitable for molding which is soft, has a low softening point which would result in a very large thermal expansion.